For years, the fields of aerospace and structural engineering have been hindered by the inability of scientists to accurately detect structural stress, to analyze the degrading effects of long-term fatigue, and, most importantly, to predict when a structure or structural member will break or fail.
Particularly in the cases of advanced composites for military aircraft and aluminum engines for high performance automobiles, systematic fatigue is nearly impossible to detect (see Case, S W., and K L. Reifsnider. Fatigue of Composite Materials. Virginia Polytechnic Institute and State University. 20 Dec. 2004. In this regard, there are usually no signs of material fatigue until the structure actually begins to break down or crumble.
Current commercial stress detection systems typically involve the use of interferometers or diffractometers, which essentially measure how much a material is compressed (see Pirling, T, and R Wimpory. Stress measurements on D1A: a new high precision strain-scanner. Institut Laue-Langevin. 17 Dec. 2004.). This technique is, in general, ineffective because all materials have a normal elastic range, and it is difficult to immediately distinguish the change from the elastic region to the plastic region (see Hookes Law—Strength (Mechanics) of Materials. Engineers Edge. 17 Dec. 2004.). Further, this method does not determine internal damage nor does it show stress levels within the interior of the material.
As designers and engineers push their creations to their design limits, the increasing stress demands for engines, aircraft, and skyscrapers spawn problems such that a single deformed bolt could lead to failure and disaster. The inability to detect material stress can effectively mean the difference between life and death.
Substantial resources have been spent on costly reconstructions and improvements. Engineers have been trying to battle the problem of material fatigue by adding reinforcements upon reinforcements to their structures, while chemists attempt to formulate new alloys and composites in hopes of finding the next ultra-light, ultra-strong alloy (see Britt, Robert R. Space Age Metal: New Titanium Alloys Near ‘Magic’ Strength Threshold. 22 Apr. 2003. Space.com. 17 Dec. 2004. <http://www.space.com/businesstechnology/technology/new_alloy—030422.html>). However, even the most advanced materials weaken and it is inevitable that repairs and replacements are needed. Thus, while developing new materials is important, accurate detection and quick replacement of damaged components are key to solving the problem of material fatigue.
As discussed below, the present invention, in accordance with one aspect thereof, concerns the phenomena of deformation luminescence and its application as an indicator of material fatigue and predictor of failure. Deformation luminescence is the emission of light produced when a regular lattice structure, such as a crystal, is plastically deformed. It appears from the literature that the effect of deformation luminescence was first discovered in the late 1950's (see Butler, C T. “Room-Temperature Deformation Luminescence in Alkali Halides.” Physical Review 141.2 (1966): 750-757). This phenomenon is not to be confused with triboluminescence, which is the emission of light from electrical sparks produced when a material is fractured or scratched (typically piezoelectric) (see Hagihara, T. “Deformation Luminescence in Gamma-Irradiated Alkali Halides.” Physics Letters A 137. (1989): 213-216).
Several studies have been conducted attempting to explain deformation luminescence (see Molotskii, M I., and S Z. Shmurak. “Elementary Acts of Deformation.” Physics Letters A 166. (1992): 286-291). The most comprehensive theory to date proposes a dislocation mechanism for deformation luminescence. According to this theory, ionizing radiation, as well as other high energy processes, dislocate electrons and ions in the regular lattice structure. Electrons are relocated into high energy electron wells known as F-centers, which color the crystal. When the crystal is plastically deformed, the stress overcomes the F-centers and subsequently ionizes them. The electrons are then released in the conduction band and dropped into a lower level well. If an electron recombines with a luminescence center, deformation luminescence is observed (see Hayashiuchi, Y. “Theory of Deformation Luminescence in KCI Crystal.” Physics Letters A 147. (1990): 245-249). According to some studies, ionizing radiation greatly amplifies photon emissions by increasing the number of F-centers within the lattice structure (see Srinivasan, M., and DeWerd, L A. “The Effect of Plastic Deformation on the Thermoluminescence of LiF (TLD-100) single crystals.” Journal of Physics D: Applied Physics 6. (1973): 2142-2149). The concentration of F centers is postulated to be exactly proportional to the intensity of deformation luminescence.